Inorganic materials such as selenium, cadmium sulfide, zinc oxide and amorphous silicon are well known as photoconductive compositions for use in electrophotographic photoreceptors.
These inorganic photoreceptors advantageously have good electrophotographic .characteristics of extremely high photoconductivity as well as excellent charge acceptability and electric insulating property in the dark. On the contrary, however, these photoreceptors have various drawbacks. For instance, a selenium photoreceptor has drawbacks that the manufacturing cost is high, moreover, the selenium photoreceptor itself is not flexible and is not resistant to heat and mechanical shock. A cadmium sulfide photoreceptor causes environmental pollution since it contains cadmium which is harmful. A zinc oxide photoreceptor has poor image stability when used repeatedly for a long period of time. An amorphous silicon photoreceptor also has the drawbacks that the manufacture cost is extremely high and a particular surface treatment for the purpose of preventing deterioration of the surface of the photoreceptor itself is necessary.
Recently, electrophotographic photoreceptors using various organic materials have been proposed so as to overcome the drawbacks of the inorganic materials, and some have been put to practical use. For example, an electrophotographic photoreceptor comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9-one (U.S. Pat. No. 3,484,237); an electrophotographic photoreceptor comprising poly-N-vinylcarbazole sensitized with a pyrylium salt dye (JP-B-No. 48-25658) (the term "JP-B" as used herein means an "examined Japanese patent publication"); and an electrophotographic photoreceptor with as a main component, an eutectic complex comprising a dye and a resin (JP-A-No. 47-10735) (the term "JP-A" as used herein means an "unexamined published Japanese patent application") are known.
In addition, electrophotographic photoreceptors with, as a main component, an organic dye such as perylene pigments (U.S. Pat. No. 3,371,884), phthalocyanine pigments (U.S. Pat. Nos. 3,397,086 and 4,666,802), azulenium salt pigments (JP-A-No. 59-53850 and JP-A-No. 61-212542), squalium salt pigments (U.S. Pat. No. 4,396,610 and 4,644,082) and polycyclic quinone pigments (JP-A-No. 59-184348 and JP-A-No. 62-28738), or an azo pigments, as described below, have widely been studied recently and various proposals have been made.